Medical system

ABSTRACT

A medical system comprising a selection unit configured to select at least one of layers in a three-dimensional tomographic image of a retina, and a generation unit configured to generate an opacity function indicating an opacity of each of a plurality of voxels constituting the selected layer, based on a frequency distribution of luminance values of the plurality of voxels.

This application is a continuation of application Ser. No. 13/231,157filed Sep. 13, 2011.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a medical system.

2. Description of the Related Art

Recently, in ophthalmic sites, an apparatus called an optical coherencetomography (to be referred to as an OCT hereinafter) has beenintroduced. This apparatus can obtain a volume image constituted by aplurality of two-dimensional tomographic images of the retina.

In ophthalmic sites, users (technicians and doctors) interpret a layerstructure from a volume image and observe the states and distribution oflesions and the three-dimensional running of fundus blood vessels. It istherefore necessary to display the distribution of lesions and bloodvessels by using volume images so as to facilitate observation.

As a technique for implementing such a display, volume rendering isknown, which assigns opacities to voxels in accordance with an opacityfunction, and performs translucent display of target volume data,thereby visualizing an internal structure.

In general, the user generates an opacity function by manually settingthe shape and the position of a peak, width, and the like of a functionby using a user interface. Japanese Patent Laid-Open No. 2008-006274discloses a technique of automatically generating such an opacityfunction. This technique fits a Gaussian function to a CT valuehistogram and can opaquely display the range of CT values calculatedfrom the average value and standard deviation of the resultant data.

In this case, the following problems arise in the arrangement configuredto automatically generate an opacity function to perform volumerendering for easy observation of tissues in a volume image of theretina.

Consider a case in which blood vessels are displayed. In this case, in atomographic image obtained by OCT, the luminance value of a blood vesselregion is high. Since a region near the lower end of the nerve fiberlayer in which blood vessels run has a high luminance value, thecontrast of the region in which blood vessels run is low. This makes itdifficult to facilitate observation of blood vessels by volume renderingusing an automatically generated opacity function.

Consider a large number of white spots distributed in a layer below thenerve fiber layer and a layer above the retinal pigment epithelium. Theluminance values of white spots are high like those of these layers. Forthis reason, even if the luminance values are converted into opacities,the nerve fiber layer and the retinal pigment epithelium located outsidethe white spots are displayed, and the white spots located inside themare difficult to display.

When displaying tissues (blood vessels and white spots) of the retinabased on a tomographic image captured by OCT in this manner, even if anopacity function is automatically generated, the tissues may not beeffectively displayed to the user (technician or doctor).

SUMMARY OF THE INVENTION

The present invention provides a technique of automatically generatingan opacity function in accordance with a display target.

According to a first aspect of the present invention there is provided amedical system comprising: a selection unit configured to select atleast one of layers in a three-dimensional tomographic image of aretina; and a generation unit configured to generate an opacity functionindicating an opacity of each of a plurality of voxels constituting theselected layer, based on a frequency distribution of luminance values ofthe plurality of voxels.

According to a second aspect of the present invention there is provideda medical system comprising: a selection unit configured to select atleast one of layers in a three-dimensional tomographic image of aretina; and a generation unit configured to generate a frequencydistribution of luminance values of a plurality of voxels constitutingthe selected layer.

Further features of the present invention will be apparent from thefollowing description of exemplary embodiments (with reference to theattached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention, andtogether with the description, serve to explain the principles of theinvention.

FIG. 1 is a block diagram showing an example of the overall arrangementof a diagnosis support system according to an embodiment of the presentinvention;

FIG. 2 is a block diagram showing an example of the functionalarrangement of an image display apparatus 20 shown in FIG. 1;

FIG. 3 is a schematic view showing an example of the layer structure ofthe retina;

FIG. 4 is a flowchart showing an example of a processing procedure inthe image display apparatus 20 shown in FIG. 1;

FIG. 5A is a graph showing an example of the distribution of luminancevalues in a selected layer;

FIG. 5B is a graph showing an example of an opacity function;

FIGS. 6A and 6B are views showing an example of a display form;

FIG. 7A is a graph showing an example of an opacity function accordingto the third embodiment; and

FIG. 7B is a view showing an example of a display form according to thethird embodiment.

DESCRIPTION OF THE EMBODIMENTS

An exemplary embodiment(s) of the present invention will now bedescribed in detail with reference to the drawings. It should be notedthat the relative arrangement of the components, the numericalexpressions and numerical values set forth in these embodiments do notlimit the scope of the present invention unless it is specificallystated otherwise.

(First Embodiment)

FIG. 1 is a block diagram showing an example of the overall arrangementof a diagnosis support system (medical system) according to anembodiment of the present invention.

A tomography apparatus 10, an image display apparatus 20, and a dataserver 30 are connected to this diagnosis support system via a network40 formed by a LAN (Local Area Network) and the like. Note that therespective apparatuses need not always be connected via the network 40as long as they can communicate with each other. For example, they canbe connected to each other via USB (Universal Serial Bus), IEEE1394, orWAN (Wide Area Network).

In this case, the tomography apparatus (optical coherence tomographyapparatus) 10 is implemented by, for example, a time-domain OCT orFourier domain OCT, and has a function of capturing a tomographic imageof the retina. The tomography apparatus 10 (OCT) obtains a plurality oftomographic images by one imaging operation, and sequentially arrangesthe tomographic images. This makes it possible to obtain a volume image(three-dimensional tomographic image) of the retina.

The tomography apparatus 10 captures tomographic images of an object(patient) and transmits the obtained volume image to the image displayapparatus 20 in accordance with the operation by the user (technician ordoctor). In some cases, this apparatus may transmit the volume imagedata to the data server 30.

The data server 30 has a function of storing various kinds of data. Thedata server 30 according to this embodiment stores the volume image ofthe retina captured by the tomography apparatus 10.

The image display apparatus 20 presents various kinds of information tothe user. More specifically, the image display apparatus 20 displays thevolume image captured by the tomography apparatus 10 or the volume imageobtained from the data server 30 to the user.

An example of the functional arrangement of the image display apparatus20 shown in FIG. 1 will be described next with reference to FIG. 2.

The image display apparatus 20 includes an input unit 51, a display unit52, a storage unit 53, a communication unit 54, and a control unit 55,which constitute the functional arrangement of the image displayapparatus 20.

The input unit 51 is implemented by, for example, a keyboard and amouse, and inputs instructions from the user (doctor or technician) intothe apparatus. The display unit 52 is implemented by, for example, adisplay such as a monitor, and displays various kinds of information tothe user. Note that the display unit 52 may be provided outside theimage display apparatus 20. That is, it is possible to use aninformation processing apparatus which performs display processing foran external display, instead of the image display apparatus. Inaddition, the input unit 51 and the display unit 52 may be implementedas a touch panel.

The storage unit 53 is implemented by, for example, a hard disk, andstores various kinds of information. The communication unit 54 isimplemented by, for example, a network card, and exchanges various kindsof data with the tomography apparatus 10 and the data server 30. Thecontrol unit 55 is implemented by a CPU, ROM (Read Only Memory), and RAM(Random Access Memory), and comprehensively controls processing in theimage display apparatus 20.

In this case, the control unit 55 includes an image obtaining unit 21,an image analysis unit 22, a display mode setting unit 23, a displaymode obtaining unit 24, a layer selection unit 25, an opacity functiongeneration unit 26, and a volume rendering unit 27. Note that eachcomponent in the control unit 55 is implemented by, for example, a CPU(Central Processing Unit) which reads out and executes programs storedin a ROM and the like.

The image obtaining unit 21 obtains the images captured by thetomography apparatus 10 and images stored in the data server via thecommunication unit 54 and the network 40. Note that the image obtainingunit 21 may directly obtain such images from an external storage medium(for example, a USB memory).

The image analysis unit 22 analyzes the volume image obtained by theimage obtaining unit 21. More specifically, the image analysis unit 22analyzes the layer structure of the retina in the volume image (in thetomographic image of the retina), and extracts the boundary of eachlayer. In this processing, as shown in FIG. 3, the image analysis unit22 extracts, for example, the inner limiting membrane (ILM), nerve fiberlayer (NFL), inner plexiform layer (IPL), outer plexiform layer (OPL),inner nuclear layer (INL), outer nuclear layer (ONL), photoreceptorinner segment/outer segment (IS/OS) junction, and retinal pigmentepithelium (RPE).

The display mode setting unit 23 sets a display mode based on aninstruction from the user via the input unit 51. In this case, thedisplay mode indicates a tissue as a display target in a volume image(in a tomographic image of the retina). A tissue as a display targetincludes, for example, a blood vessel, white spot, or cyst.

The display mode obtaining unit 24 obtains the display mode set by thedisplay mode setting unit 23. Display modes include, for example, ablood vessel mode (in which a tissue as a display target is a bloodvessel), a white spot mode (in which a tissue as a display target is awhite spot), and a cyst mode (in which a tissue as a display target is acyst). Obviously, it is possible to set other types of display modes.

The layer selection unit 25 selects a layer to be used to generate anopacity function based on the display mode set by the display modesetting unit 23 and the boundary of each layer extracted by the imageanalysis unit 22. If, for example, a tissue as a display target is ablood vessel (that is, the blood vessel mode), since the running ofblood vessels are often seen in the inner plexiform layer (IPL), thelayer selection unit 25 selects the IPL.

The opacity function generation unit 26 generates (different) opacityfunctions based on the layer selected by the layer selection unit 25.More specifically, the opacity function generation unit 26 generates anopacity function that assigns high opacities to voxels corresponding tothe tissue as a display target and assigns low opacities to voxelscorresponding to other tissues, based on the luminance values of aplurality of voxels in the layer selected by the layer selection unit25. An opacity function is a function that changes a feature amount (forexample, a luminance value) of each voxel into an opacity. For example,transparency is expressed by “0.0”, and opacity is expressed by “1.0”.

The volume rendering unit 27 performs volume rendering of a volume imageon the display (input/output unit) by using the opacity functiongenerated by the opacity function generation unit 26. This can providedisplay that allows the user to easily observe the tissue as a displaytarget.

An example of a processing procedure in the image display apparatus 20shown in FIG. 1 will be described next with reference to FIG. 4. Thefollowing will exemplify operation to be performed when the blood vesselmode (a tissue as a display target is a blood vessel) is set.

When starting this processing, first of all, the image display apparatus20 causes the image obtaining unit 21 to obtain the volume imagecaptured by the tomography apparatus 10 or a volume image stored in thedata server 30 (S101). The image analysis unit 22 analyzes the layerstructure of the retina in the obtained volume image and extracts theboundary of each layer (S102).

The image display apparatus 20 then causes the layer selection unit 25to obtain the display mode designated by the user. The layer selectionunit 25 selects a layer to be used to generate an opacity function inaccordance with the obtained display mode (S103). When, for example, theuser wants to display a blood vessel, he/she sets the blood vessel modeas a display mode. In this case, the layer selection unit 25 sets theIPL in the blood vessel mode because the running of blood vessels isoften seen in the IPL.

Upon selection of a layer, the image display apparatus 20 causes theopacity function generation unit 26 to generate an opacity functionnecessary for volume rendering (S104). When generating an opacityfunction, first of all, the opacity function generation unit 26generates a luminance value histogram of a plurality of voxelsconstituting the layer selected in the processing in step S103 andcalculates the average value and standard deviation of the histogram. Byusing the calculated average value and standard deviation, the opacityfunction generation unit 26 generates an opacity function that makesvoxels with lower luminance values become more transparent, and makesvoxels with higher luminance values become more opaque.

In this case, since the blood vessel mode is set, the opacity functiongeneration unit 26 generates a luminance value histogram of voxels inthe IPL (see FIG. 5A), and calculates the average value and standarddeviation of the histogram. Note that voxels in the IPL are thoselocated between the lower end of the NFL and the lower end of the IPLwhich are obtained as an extraction result by the image analysis unit22. In OCT, high luminance values appear in a region considered as aregion in which a blood vessel runs. For this reason, it is thought thata large number of tissues other than blood vessels are distributed in arange in which luminance values are lower than a first reference value A(in this case, the average value) of the histogram shown in FIG. 5A. Itis also thought that a large number of blood vessel tissues aredistributed in the range of luminance values between the first referencevalue A (average value) and a second reference value B (averagevalue+n×standard deviation).

To display the information between A and B so as to facilitateobservation of the information, the opacity function generation unit 26generates the opacity function shown in FIG. 5B. More specifically, theopacity function generation unit 26 generates an opacity function thatmakes regions (voxels) with luminance values equal to or less than thefirst reference value (average value) transparent and makes regions withluminance values exceeding the second reference value (averagevalue+n×standard deviation) opaque. In addition, this opacity functionis configured to increase the opacity with an increase in luminancevalue between A and B. That is, voxels having luminance values between Aand B are processed into translucent voxels. In this case, the value nhas a function of controlling the range of luminance values in whichvoxels are made translucent.

According to an example of the opacity function shown in FIG. 5B, themanner of changes in opacity is indicated as linear. Obviously, theapparatus may be configured to generate a nonlinear function such as aquadratic function. Using a quadratic function will assign loweropacities to regions with lower luminance values and assigns higheropacities to regions with higher luminance values. This can display ablood vessel region with higher contrast.

The image display apparatus 20 then causes the volume rendering unit 27to perform volume rendering of the volume image (S105). The volumerendering unit 27 performs this processing by using the layer extractionresult in step S102, the opacity function generated in step S104, andthe layer selected in step S103. As a volume rendering algorithm, agenerally known volume ray casting method can be used. Note, however,that target voxels from which opacities are calculated based onluminance values are voxels in a layer selected based on the bloodvessel mode. Voxels other than the target voxels are assigned with theopacity “0.0”, that is, are made transparent.

As described above, according to this embodiment, when, for example, theuser has selected the blood vessel mode, this apparatus automaticallygenerates an opacity function that assigns high opacities to bloodvessel tissues and assigns low opacities to tissues other than the bloodvessel tissues. Performing volume rendering by using this opacityfunction can present the display shown in FIG. 6A to the user. Thisallows the user to easily observe three-dimensional running of bloodvessels running in the retinal layer. Note that FIG. 6A is a schematicview displaying three-dimensional running of blood vessels viewed fromabove (y direction) the retina shown in FIG. 3.

(Second Embodiment)

The second embodiment will be described next. The second embodiment willexemplify operation to be performed upon setting of the white spot modeof performing volume rendering of a three-dimensional distribution ofwhite spots. Note that since the arrangements of a diagnosis supportsystem and image display apparatus 20 according to the second embodimentare the same as those shown in FIGS. 1 and 2, with reference to whichthe first embodiment has been described, a description of them will beomitted. Different processes from those in the first embodiment will bemainly described with reference to the flowchart of FIG. 4, withreference to which the first embodiment has been described.

The image display apparatus 20 obtains a volume image, and then analyzesthe volume image as in the first embodiment described above (steps S101and S102).

In this case, if the white spot mode is set, the image display apparatus20 causes a layer selection unit 25 to select the layer located betweenthe lower end of the NFL and the IS/OS because a large number of whitespots are distributed in the layer between them (S103).

The image display apparatus 20 then causes an opacity functiongeneration unit 26 to generate a luminance value histogram of aplurality of voxels constituting the layer selected in the processing instep S103, and calculates the average value and standard deviation ofthe histogram. By using the calculated average value and standarddeviation, the opacity function generation unit 26 generates an opacityfunction that makes voxels with lower luminance values become moretransparent, and makes voxels with higher luminance values become moreopaque (S104). Note that the voxels used to generate the luminance valuehistogram are those located between the lower end of the NFL and IS/OS.In OCT, the luminance value of a white spot is high. For this reason, itis thought that in the luminance value histogram, a large number ofwhite spots are distributed in the range of high luminance values, and alarge number of tissues other than the white spots are distributed inthe range of low luminance values. The opacity function generation unit26 generates an opacity function that makes voxels with luminance valuesequal to or less than a first reference value A (average value)transparent and makes voxels with luminance values exceeding a secondreference value B (average value+n×standard deviation) opaque as in thefirst embodiment. In addition, the opacity function generation unit 26generates an opacity function that changes the opacities of voxelshaving luminance values between the reference values A and B as in thefirst embodiment.

The image display apparatus 20 then causes a volume rendering unit 27 toperform volume rendering of the volume image as in the first embodiment(S105). In this case, assume that voxels for which opacities are to becalculated from luminance values are those in the layer selected basedon the white spot mode. Voxels other than the target voxels are assignedwith the opacity “0.0”, that is, are made transparent.

As described above, according to the second embodiment, when the userhas selected the white spot mode, this apparatus automatically generatesan opacity function that assigns high opacities to white spot tissues,and assigns low opacities to tissues other than the white spot tissues.Performing volume rendering by using this opacity function can presentthe display shown in FIG. 6B to the user. This allows the user to easilyobserve three-dimensional distributions of white spots andhigh-luminance tissues in the retinal layer.

(Third Embodiment)

The third embodiment will be described next. The third embodiment willexemplify operation to be performed when performing volume rendering ofa three-dimensional distribution of cysts. Note that since thearrangements of a diagnosis support system and image display apparatus20 according to the third embodiment are the same as those shown inFIGS. 1 and 2, with reference to which the first embodiment has beendescribed, a description of them will be omitted. Different processesfrom those in the first embodiment will be mainly described withreference to the flowchart of FIG. 4, with reference to which the firstembodiment has been described.

When the white spot mode or the blood vessel mode is set, this apparatusgenerates an opacity function that assigns high opacities to regions(voxels) with high luminance values, and assigns low opacities toregions with low luminance values. In contrast to this, in the cystmode, when performing OCT, the apparatus generates an opacity functionthat assigns high opacities to regions with low luminance values, andassigns low opacities to regions with high luminance values, becausecyst regions have low luminance values.

If the cyst mode is set, the image display apparatus 20 causes a layerselection unit 25 to select at least one of the INL, OPL, and ONLbecause a large number of cysts are distributed in them (S103).

The image display apparatus 20 then causes an opacity functiongeneration unit 26 to generate a luminance value histogram of aplurality of voxels constituting the layer selected in the processing instep S103, and calculates the average value and standard deviation ofthe histogram. By using the calculated average value and standarddeviation, the opacity function generation unit 26 generates an opacityfunction that makes voxels with lower luminance values become moretransparent, and makes voxels with higher luminance values become moreopaque (S104). Note that the voxels used to generate a luminance valuehistogram are those located between the upper boundary of the INL andthe lower boundary of the ONL. As described above, since the luminancevalue of a cyst region is low, it is necessary to display the regionwith low luminance so as to facilitate observation of the region.

As shown in FIG. 7A, the opacity function generation unit 26 thereforegenerates an opacity function that makes voxels with luminance valuesequal to or less than a first reference value A (in this case, averagevalue−n×standard deviation) opaque, and makes voxels with luminancevalues exceeding a second reference value B (average value) transparent.This opacity function is also configured to assign lower opacities tovoxels with luminance values between A and B (voxels having luminancevalues which exceed the first reference value and are equal to or lessthan the second reference value) with an increase in luminance value.That is, voxels having luminance values between A and B are processedinto translucent voxels. As in the above case, the value n has afunction of controlling the range of luminance values in which voxelsare made translucent.

The image display apparatus 20 then causes the volume rendering unit 27to perform volume rendering of a volume image as in the first embodiment(S105).

As described above, according to the third embodiment, when the user hasselected the cyst mode, this apparatus automatically generates anopacity function that assigns high opacities to white cyst tissues, andassigns low opacities to tissues other than the cyst tissues. Performingvolume rendering by using this opacity function can present the displayshown in FIG. 7B to the user. This allows the user to easily observethree-dimensional distributions of cysts and high-luminance tissues inthe retinal layer.

The typical embodiments of the present invention have been describedabove. However, the present invention is not limited to the embodimentsdescribed above and shown in the accompanying drawings, and can bemodified and executed as needed within the spirit and scope of theinvention.

For example, tissues as display targets in a tomographic image of theretina may be the entire retina. In this case, the image displayapparatus 20 generates an opacity function from the voxels in the entireretinal layer, and performs volume rendering by using the opacityfunction. This makes it possible to automatically display the entireretina so as to facilitate observation of the retina.

As has been described above, according to the present invention, anopacity function is automatically generated in accordance with a displaytarget. This makes it possible to perform volume rendering that allowsthe user to easily observe a tissue as a display target, therebyimproving diagnosis accuracy.

(Other Embodiments)

Aspects of the present invention can also be realized by a computer of asystem or apparatus (or devices such as a CPU or MPU) that reads out andexecutes a program recorded on a memory device to perform the functionsof the above-described embodiment(s), and by a method, the steps ofwhich are performed by a computer of a system or apparatus by, forexample, reading out and executing a program recorded on a memory deviceto perform the functions of the above-described embodiment(s). For thispurpose, the program is provided to the computer for example via anetwork or from a recording medium of various types serving as thememory device (for example, computer-readable storage medium).

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2010-219765 filed on Sep. 29, 2010, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. A processing apparatus comprising: an analysisunit configured to analyze a layer structure of a three-dimensionaltomographic image of a retina acquired by an optical coherencetomography apparatus; a selection unit configured to select a displaytarget other than the analyzed layer structure in a retina; and ageneration unit configured to generate an opacity function indicating anopacity of a plurality of voxels constituting a region corresponding tothe selected display target in a three-dimensional tomographic image ofthe retina, based on the selected display target and a frequencydistribution of luminance values of the plurality of voxels.
 2. Theapparatus according to claim 1, wherein said generation unit generates,based on the selected display target, an opacity function that indicateshigher or lower opacities for voxels, which have luminance values higherthan a predetermined value, than for other voxels among the plurality ofvoxels.
 3. The apparatus according to claim 1, further comprising: avolume rendering unit configured to perform volume rendering of theregion based on the opacity function; and a display control unitconfigured to cause a display unit to display an image having undergonethe volume rendering.
 4. The apparatus according to claim 1, wherein theselection unit selects at least one of layers corresponding to theselected display target as the region, and wherein the generation unitgenerates the opacity function based on the selected display target anda frequency distribution of luminance values of the plurality of voxelsconstituting the selected layers.
 5. The apparatus according to claim 4,further comprising a setting unit configured to set a display modeindicating the display target, wherein the selection unit selects, inaccordance with the set display mode, the at least one of layers as theregion.
 6. The apparatus according to claim 1, wherein the selectionunit selects an inner plexiform layer as the region when a blood vesselis selected as the display target, and wherein the generation unitgenerates, based on a frequency distribution of luminance values of theplurality of voxels constituting the selected inner plexiform layer, anopacity function that indicates higher opacities for voxels, which haveluminance values higher than a predetermined value, than for othervoxels among the plurality of voxels.
 7. The apparatus according toclaim 1, wherein the selection unit selects layers between a lower endof a nerve fiber layer and a retinal pigment epithelium as the regionwhen a white spot is selected as the display target, and wherein thegeneration unit generates, based on a frequency distribution ofluminance values of the plurality of voxels constituting the selectedlayers, an opacity function that indicates higher opacities for voxels,which have luminance values higher than a predetermined value, than forother voxels among the plurality of voxels.
 8. The apparatus accordingto claim 1, wherein the selection unit selects an inner nuclear layer,an external pexiform layer, and an outer nuclear layer as the regionwhen a cyst is selected as the display target, and wherein thegeneration unit generates, based on a frequency distribution ofluminance values of the plurality of voxels constituting the selectedlayers, an opacity function that indicates lower opacities for voxels,which have luminance values higher than a predetermined value, than forother voxels among the plurality of voxels.
 9. The apparatus accordingto claim 1, wherein said generation unit (a) calculates a firstreference value for luminance values of the plurality of voxels and asecond reference value indicating a larger value than the firstreference value by using an average value and standard deviationcalculated based on the luminance values, and (b) generates an opacityfunction that makes a voxel having a luminance value not more than thefirst reference value transparent, makes a voxel having a luminancevalue exceeding the second reference value opaque, and increases anopacity of a voxel having a luminance value which exceeds the firstreference value and is not more than the second reference value, with anincrease in luminance value from the first reference value to the secondreference value.
 10. The apparatus according to claim 4, wherein saidgeneration unit (a) calculates a first reference value for luminancevalues of the plurality of voxels in the selected layers and a secondreference value indicating a larger value than the first reference valueby using an average value and standard deviation calculated based on theluminance values, and (b) generates an opacity function that makes avoxel having a luminance value not more than the first reference valueopaque, makes a voxel having a luminance value exceeding the secondreference value transparent, and increases a transparency of a voxelhaving a luminance value which exceeds the first reference value and isnot more than the second reference value, with an increase in luminancevalue from the first reference value to the second reference value. 11.A processing method comprising: analyzing a layer structure of athree-dimensional tomographic image of a retina acquired by an opticalcoherence tomography apparatus; selecting a display target other thanthe analyzed layer structure in a retina; and generating an opacityfunction indicating an opacity of a plurality of voxels constituting aregion corresponding to the selected display target in athree-dimensional tomographic image of the retina, based on the selecteddisplay target and a frequency distribution of luminance values of theplurality of voxels.
 12. The method according to claim 11, wherein insaid step of generating, based on the selected display target, anopacity function is generated that indicates higher or lower opacitiesfor voxels, which have luminance values higher than a predeterminedvalue, than for other voxels among the plurality of voxels.
 13. Themethod according to claim 11, further comprising: performing volumerendering of the region based on the opacity function; and causing adisplay unit to display an image having undergone the volume rendering.14. The method according to claim 11, wherein in the step of selecting,at least one of layers corresponding to the selected display target areselected as the region, and wherein in the step of generating, theopacity function is generated based on the selected display target and afrequency distribution of luminance values of the plurality of voxelsconstituting the selected layers.
 15. The method according to claim 14,further comprising setting a display mode indicating the display target,wherein in the step of selecting, in accordance with the set displaymode, the at least one of layers are selected as the region.
 16. Themethod according to claim 11, wherein in the step of selecting, an innerplexiform layer is selected as the region when a blood vessel isselected as the display target, and wherein in the step of generating,based on a frequency distribution of luminance values of the pluralityof voxels constituting the selected inner plexiform layer, an opacityfunction is generated that indicates higher opacities for voxels, whichhave luminance values higher than a predetermined value, than for othervoxels among the plurality of voxels.
 17. The method according to claim11, wherein in the step of selecting, layers between a lower end of anerve fiber layer and a retinal pigment epithelium are selected as theregion when a white spot is selected as the display target, and whereinin the step of generating, based on a frequency distribution ofluminance values of the plurality of voxels constituting the selectedlayers, an opacity function is generated that indicates higher opacitiesfor voxels, which have luminance values higher than a predeterminedvalue, than for other voxels among the plurality of voxels.
 18. Themethod according to claim 11, wherein in the step of selecting, an innernuclear layer, an external pexiform layer, and an outer nuclear layerare selected as the region when a cyst is selected as the displaytarget, and wherein in the step of generating, based on a frequencydistribution of luminance values of the plurality of voxels constitutingthe selected layers, an opacity function is generated that indicateslower opacities for voxels, which have luminance values higher than apredetermined value, than for other voxels among the plurality ofvoxels.
 19. A non-transitory computer-readable storage medium storing aprogram for causing a computer to execute each step of a processingmethod defined in claim
 11. 20. A medical system comprising: an analysisunit configured to analyze a layer structure of a three-dimensionaltomographic image of a retina acquired by an optical coherencetomography apparatus; a layer selection unit configured to select atleast one layer of layers in the three-dimensional tomographic image ofthe retina by using the analyzed layer structure based on a displaytarget other than the analyzed layer structure in the three-dimensionaltomographic image of the retina; a generation unit configured togenerate an opacity function indicating an opacity of each of aplurality of voxels constituting the selected layer, based on afrequency distribution of luminance values of the plurality of voxels;and a volume rendering unit configured to perform volume rendering ofthe three-dimensional tomographic image of the retina based on theopacity function.
 21. A processing apparatus comprising: an analysisunit configured to analyze a layer structure of a three-dimensionaltomographic image of a retina acquired by an optical coherencetomography apparatus; a layer selection unit configured to select atleast one layer of layers in the three-dimensional tomographic image ofthe retina by using the analyzed layer structure based on a displaytarget other than the analyzed layer structure in the three-dimensionaltomographic image of the retina; a generation unit configured togenerate an opacity function indicating an opacity of each of aplurality of voxels constituting the selected layer, based on afrequency distribution of luminance values of the plurality of voxels;and a volume rendering unit configured to perform volume rendering ofthe three-dimensional tomographic image of the retina based on theopacity function.
 22. A processing method comprising: analyzing a layerstructure of a three-dimensional tomographic image of a retina acquiredby an optical coherence tomography apparatus; selecting at least onelayer of layers in the three-dimensional tomographic image of the retinaby using the analyzed layer structure based on a display target otherthan the analyzed layer structure in the three-dimensional tomographicimage of the retina; generating an opacity function indicating anopacity of each of a plurality of voxels constituting the selectedlayer, based on a frequency distribution of luminance values of theplurality of voxels; and performing volume rendering of thethree-dimensional tomographic image of the retina based on the opacityfunction.
 23. A non-transitory computer-readable storage medium storinga program for causing a computer to execute each step of a processingmethod defined in claim
 22. 24. A processing apparatus comprising: ananalysis unit configured to analyze a layer structure of athree-dimensional tomographic image of a retina acquired by an opticalcoherence tomography apparatus; a layer selection unit configured toselect at least one layer of layers in the three-dimensional tomographicimage of the retina by using the analyzed layer structure; and a volumerendering unit configured to perform volume rendering of thethree-dimensional tomographic image of the retina based on an opacityfunction indicating an opacity of a plurality of voxels constituting theselected layer.
 25. A processing method comprising: analyzing a layerstructure of a three-dimensional tomographic image of a retina acquiredby an optical coherence tomography apparatus; selecting at least onelayer of layers in the three-dimensional tomographic image of the retinaby using the analyzed layer structure; and performing volume renderingof the three-dimensional tomographic image of the retina based on anopacity function indicating an opacity of a plurality of voxelsconstituting the selected layer.
 26. A non-transitory computer-readablestorage medium storing a program for causing a computer to execute eachstep of a processing method defined in claim 25.